@jro1 A lot of people are posting advice based on feelings and ego. I worked in payroll. Let's talk $.
How much is her hourly fee? Are we talking 15/hr or 100?
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This is a business. If she's bad at estimating and on the low end and is taking clients this late, I'd eat it and say absolutely nothing. If you have no backup plan, I'd eat it and say absolutely nothing. Because you're not finding someone else this late and if you do they're going to price-gouge too. You're doing this waaaaay too late in the year. Your returns are due in 30 days and you're just looking over last year's books? JFC, c'mon. A late return is $200+/mth. You might be able to file an extension, but IDK if you have any grounds.
QB has a GL IDK what needs balancing. Considering that the fact you're using modern tech and your books "need balancing" scares me, I wouldn't be too hasty on trying to annoy this person if it's not a lot of money and is just ego. They may drop you as a client.
Your worst case is actually not paying +4 hours, your worst case is they refund you completely, nullifying the contract, and just hand you your books back. A fair number of people will do this if you annoy them sufficiently at year end because they are going to be working with multiple clients, you're probably not a big one, and it's not worth the effort of hassling over it.
Not a lawyer not legal or business advice.